1. Field of the Invention
The present invention relates to a liquid crystal display and, more specifically, to an improvement of a liquid crystal display including non-linear MIM (Metal-Insulator-Metal) elements between pixel electrodes and signal lines.
2. Description of the Background Art
Recently, non-linear MIM elements which can be fabricated relatively easily are used in liquid crystal display for high multiplexing matrix drive (highly time-sharing matrix addressing) of liquid crystal display cells.
FIG. 5 is a cross section showing one example of the conventional MIM element. In this figure, a signal line 112 of tantalum or aluminum is formed on a transparent insulating substrate 111 formed of, for example, glass. A portion 121 of signal line 112 is also used as a first metal layer of the MIM element. Signal line 112 is covered with an insulating layer 122 formed of a metal oxide such as tantalum oxide or alumina. A portion of the insulating layer 122 is further covered with a second metal layer 123 of the MIM element. The second metal layer 123 can be formed of chromium or aluminum. In other words, the MIM element is comprised of the first metal layer 121, the insulating layer 122 and the second metal layer 123. The second metal layer 123 is extended to a pixel electrode 113 formed on the substrate 111 to be connected thereto.
The insulating layer 122 can be formed relatively easily by oxidizing the surface of signal line 112. However, the current-voltage (I-V) characteristic of the MIM element including the insulating layer 122 formed in that manner does not easily match the high multiplexing drive characteristic of the liquid crystal display cell.
When a liquid crystal cell is driven in high multiplexing manner, a certain AC OFF voltage is applied to the signal line even when it is not selected. In this case, it is desirable that the high ON voltage applied to the signal line surely aligns the liquid crystal molecules in the direction of the electric field. When the low OFF voltage is being applied to the signal line, the effective voltage applied to the liquid crystal should be as small as possible. However, it is difficult to make the OFF current flowing through the liquid crystal during the OFF voltage period sufficiently small, by controlling the degree of oxidation of the surface of the first metal layer and by controlling the density of the oxide film formed by such oxidation in the MIM element.
If the current flowing through the liquid crystal is not sufficiently small with the OFF voltage of about 4 V, for example, high ripples may appear in the effective voltage applied to the liquid crystal during the OFF voltage period, which ripples may possibly cause erroneous display. Charges stored by the ON voltage in a selected pixel during one scan cycle disappear too fast in the OFF voltage period, and the after image cannot be maintained till the next scan cycle, resulting in flickers on the display. It may be the reason why the OFF current cannot be made sufficiently small when the low OFF voltage is applied, that the energy barrier of the insulating film formed of, e.g., tantalum oxide, is made lower to provide a low resistance value of the MIM element with respect to a high voltage in order to align surely the liquid crystal molecules in the direction of the electric field when the ON voltage is applied, while such MIM element cannot exhibit sufficiently high resistance against the low OFF voltage.
More specifically, the MIM element used for driving the liquid crystal display cell in a high multiplexed manner must have a steep I-V characteristic.
The I-V characteristic of an MIM element is given by: EQU I=KVexp(.beta..sqroot.V) (1)
where I is current, V is voltage, and K and .beta. are represented by: ##EQU1##
In formulas (2) and (3), n is the electron density, e is the charge, .mu. is the mobility, S is area, t is the thickness of the insulator film, .phi. is the donor level, .kappa. is the Bolzmann constant, T is temperature, .SIGMA..sub.0 is the dielectric constant under vacuum and .SIGMA. is permitivity of the insulator.
As can be understood from the Pool-Frenkel's formula (1), the I-V characteristic of the MIM element becomes steeper when the value of .beta. becomes larger. Also, as can be seen from the formula (3), the value of .beta. becomes larger as the insulating layer in the MIM element becomes thinner. However, it is difficult to uniformly form a very thin insulating layer, and if the insulating layer is made too thin, the I-V characteristic of the MIM element tends to vary. When a relatively thick insulating layer having the thickness of 30 nm or more has been used, the value of .beta. is about 3.0, which is not sufficiently large. Accordingly, it is difficult to obtain sufficiently high contrast of display in the liquid crystal display.
In addition, the I-V characteristic of the conventional MIM element is assymetrical with respect to the change of polarity of the voltage. Therefore, when the liquid crystal cells are driven by an alternating signal, a direct current component remains in the liquid crystal cells, shortening the life of the liquid crystal display.
It is known in the prior art that a small amount of nitrogen may be contained in the first metal layer of the MIM element. However, nitrogen is contained in the first metal layer in order to reduce the resistance value of the first metal layer which is formed of tantalum, for example, and not to adjust the characteristic of the MIM element suitable for driving the liquid crystal cells.